Things to Know


#Vascular Plants/Traits




#Homospory vs Heterospory




#Plant Phylogeny

First plant group: non-vascular bryophytes (mosses)


  • Mosses, hornworts and liverworts
    • Fossils of bryophyte spores ~470mya


  • Non-vascular; ground hugging carpets
    • bodies to thin to support height growth


  • Have a rhizoid but not a root
    • anchors plant
    • does not uptake water


  • Very resistant spores

Bryophytes have a gametophyte dominated life cycle

Early plants, like moss, have sperm that swim from antheridia to archegonia (need water)

Mosses, homosporous AoG with gametophyte dominance

Vascular plants were the next evolutionary step…Why?

Vascular tissues: tissues for conducting nutrients and fluid internally

The big picture: Vascular plants now dominate Earth


  • Bryophytes ruled for <100 million years
    • vascular plants show up ~425mya


  • Plants then evolved vascular tissues
    • light competition = selection pressure
    • tissues connected leaves to roots


  • First vascular plants lacked seeds
    • sperm still needed water to swim
    • lycophytes and ferns = 1st groups


  • Fern and lycophyte diversity now diminished

Ancient seedless forests now power our planet: Coal!

Vascular system: the basics (add to phylogeny)


Xylem = water conducting cells (one way)
Phloem = sugar sap conducting cells (multi-directional)


  • Conducting cells also provide biomechanical support
    • lignin in cell walls
    • does not rot easily
    • does not collapse easily
    • ‘think’ bundles of rigid pipes
    • allows plants to grow vertically


  • Vascular tissues move liquid under tension


  • Vascular tissues evolved independently several times

Evolution of roots (add to phylogeny)

  • Vascular tissues belowground
    • extension of ancient stems
    • bryophytes have no ‘true’ roots


  • Absorb water and nutrients
    • active explorers of soil
    • most elements for plant growth are in soil
    • often in symbiosis with fungi


  • Anchor the plant in place
    • support vertical growth


  • Fossils show roots in lycophytes ~ 400mya
    • convergent evolution in ferns

Evolution of leaves (add to phylogeny)


  • Photosynthetic organ on plants
    • shape generally maximizes surface area exposed to sunlight


  • Microphylls: leaves with single unbranched vein
    • vein: leaf structure containing xylem + phloem
    • unique to lycophytes (add to phylogeny)


  • Megaphylls: leaves with many branched veins
    • majority of vascular plants (add to phylogeny)


  • Why do more veins matter?

Evolution of leaves: Sporangium

  • Sporophyll: modified leaf with sporangium
    • ‘fertile’ leaf (w/ spores)


  • Sporangium: spore container (house)
    • ferns: clusters of sporagnia called ‘sori’
    • lycophytes: ‘stobilus’ holds many sporagnia (like a cone)
    • angiosperms: structures inside of flowers
    • exception - moss have sporangia but not leaves


  • Spore mother cell: cell inside sporagnium
    • diplod cell (2N) undergoes meiosis to make a spore (1N)


  • What is the ploidy of each cell type in this picture?

Seedless vascular plants have free living separate generations


  • Sporophyte: The fern you see in the woods
    • now the dominant stage of the lifecycle


  • Gametophye: independent haploid generation
    • start out female or bisexual


  • Dependent embryo (2N) develops and replaces gametophyte
    • in moss, sporophyte develops and dies on the living gametophyte


  • What does natural selection favor diploid dominant lifecycles?

Homosphorous life cycle (mostly) with dominant sporophytes

Sporophytes are larger and long lived, gametophytes are smaller and shorter lived, each spore produces a bisexual gametophyte

Lycophytes: First vascular plants



  • Independent, branched sporophyte


  • Lignified vascular tissue, tracheids & roots
    • tracheid = long, twisted xylem cell


  • Leaves are microphylls
    • fertile leaves have strobili (spore house)


  • Some with heterospory (Selaginella spp)
    • male (micro-) and female (mega-) spores
    • male (micro-) and female (mega-) gametophytes

Ferns


  • Whisk Ferns, Equisetum & Ferns
    • mostly homosporous


  • Homospory = spores that germinate to produce bisexual (both male and female) gametophytes


  • Leaves are mostly megaphylls
    • whisk ferns lost leaves
    • Equistem has reduced megaphylls (tiny leaves)

Ferns


  • Whisk Ferns, Equisetum & Ferns
    • mostly homosporous
    • whisk ferns do not have roots


  • Leaves are mostly megaphylls
    • whisk ferns lost leaves
    • Equistem has reduced leaves


  • New Gaga genus with 19 species
    • “We wanted to name this genus for Lady Gaga because of her fervent defense of equality and individual expression” Pryer @ Duke University

The consequences of homospory



  • One spore → one gametophyte → bisexual gametophyte
    • everything is haploid (1N)


  • Produces both sperm and egg via mitosis from haploid cells
    • sperm must swim to the egg


  • A lot of self fertilization
    • what is good/bad about this?

How ferns avoid self-fertilization…


  • Fastest developing gametophyte (female) releases chemical hormone


  • Hormone turns other developing gametophytes all male
    • sperm from everywhere!
    • female power - makes her own boyfriends


  • Archegonia (F) and Antheridia (M) can also develop at different times
    • if fertilization doesn’t happen, female gametophyte becomes bisexual